Methods of making semiconductor x-ray detector

ABSTRACT

Disclosed herein is an apparatus comprising: an X-ray absorption layer; a first electrical contact and a second electrical contact on opposing surfaces of the X-ray absorption layer; wherein the first electrical contact and the second electrical contact respectively comprise structures extending into the X-ray absorption layer.

TECHNICAL FIELD

The disclosure herein relates to X-ray detectors, particularly relatesto methods of making semiconductor X-ray detectors.

BACKGROUND

X-ray detectors may be devices used to measure the flux, spatialdistribution, spectrum or other properties of X-rays.

X-ray detectors may be used for many applications. One importantapplication is imaging. X-ray imaging is a radiography technique and canbe used to reveal the internal structure of a non-uniformly composed andopaque object such as the human body.

Early X-ray detectors for imaging include photographic plates andphotographic films. A photographic plate may be a glass plate with acoating of light-sensitive emulsion. Although photographic plates werereplaced by photographic films, they may still be used in specialsituations due to the superior quality they offer and their extremestability. A photographic film may be a plastic film (e.g., a strip orsheet) with a coating of light-sensitive emulsion.

In the 1980s, photostimulable phosphor plates (PSP plates) becameavailable. A PSP plate may contain a phosphor material with colorcenters in its lattice. When the PSP plate is exposed to X-ray,electrons excited by X-ray are trapped in the color centers until theyare stimulated by a laser beam scanning over the plate surface. As theplate is scanned by laser, trapped excited electrons give off light,which is collected by a photomultiplier tube. The collected light isconverted into a digital image. In contrast to photographic plates andphotographic films, PSP plates can be reused.

Another kind of X-ray detectors are X-ray image intensifiers. Componentsof an X-ray image intensifier are usually sealed in a vacuum. Incontrast to photographic plates, photographic films, and PSP plates,X-ray image intensifiers may produce real-time images, i.e., do notrequire post-exposure processing to produce images. X-ray first hits aninput phosphor (e.g., cesium iodide) and is converted to visible light.The visible light then hits a photocathode (e.g., a thin metal layercontaining cesium and antimony compounds) and causes emission ofelectrons. The number of emitted electrons is proportional to theintensity of the incident X-ray. The emitted electrons are projected,through electron optics, onto an output phosphor and cause the outputphosphor to produce a visible-light image.

Scintillators operate somewhat similarly to X-ray image intensifiers inthat scintillators (e.g., sodium iodide) absorb X-ray and emit visiblelight, which can then be detected by a suitable image sensor for visiblelight. In scintillators, the visible light spreads and scatters in alldirections and thus reduces spatial resolution. Reducing thescintillator thickness helps to improve the spatial resolution but alsoreduces absorption of X-ray. A scintillator thus has to strike acompromise between absorption efficiency and resolution.

Semiconductor X-ray detectors largely overcome this problem by directconversion of X-ray into electric signals. A semiconductor X-raydetector may include a semiconductor layer that absorbs X-ray inwavelengths of interest. When an X-ray photon is absorbed in thesemiconductor layer, multiple charge carriers (e.g., electrons andholes) are generated and swept under an electric field towardselectrical contacts on the semiconductor layer. Cumbersome heatmanagement required in currently available semiconductor X-ray detectors(e.g., Medipix) can make a detector with a large area and a large numberof pixels difficult or impossible to produce.

SUMMARY

Disclosed herein is a method of making an apparatus suitable fordetecting x-ray, the method comprising: attaching a chip comprising anX-ray absorption layer to a surface of a substrate, wherein the surfaceis electrically conductive; thinning the chip; forming an electricalcontact in the chip; bonding an electronic layer to the chip such thatthe electrical contact of the chip is electrically connected to anelectrical contact of the electronic layer.

According to an embodiment, the substrate is not removed from the chip.

According to an embodiment, the substrate has a mass attenuationcoefficient less than 1000 m²/kg for X-ray.

According to an embodiment, the substrate comprises silicon, glass,silicon oxide, Al, Cr, Ti, or a combination thereof.

According to an embodiment, thinning comprises reducing a thickness ofthe chip to 200 microns or less, 100 microns or less, or 50 microns orless.

According to an embodiment, the chip comprises GaAs.

According to an embodiment, the GaAs is not doped with chromium.

According to an embodiment, attaching the chip to the surface of thesubstrate comprises depositing a layer of metal to the chip.

According to an embodiment, the method further comprises forming a diodein the chip.

According to an embodiment, the method further comprises formingdiscrete electrical contacts on the chip.

Disclosed herein is a method of making an apparatus suitable fordetecting x-ray, the method comprising: attaching a chip comprising anX-ray absorption layer to a surface of a substrate, wherein the surfaceis electrically conductive, wherein the chip comprises a sacrificialsubstrate, a second doped region on the sacrificial substrate; exposingthe second doped region by removing the sacrificial substrate; formingan electrical contact on the second doped region; bonding an electroniclayer to the chip such that the electrical contact of the chip iselectrically connected to an electrical contact of the electronic layer.

According to an embodiment, the method further comprises an etch stoplayer between the sacrificial substrate and the second doped region.

According to an embodiment, the method further comprises an intrinsicregion, wherein the intrinsic region and the sacrificial substratesandwich the second doped region.

According to an embodiment, the intrinsic region has a thickness of lessthan 75 microns, less than 100 microns, less than 125 microns, or lessthan 200 microns.

According to an embodiment, the method further comprises a lightly dopedregion, wherein the lightly doped region and the sacrificial substratesandwich the second doped region.

According to an embodiment, the lightly doped region has a thickness ofless than 75 microns, less than 100 microns, less than 125 microns, orless than 200 microns.

According to an embodiment, the second doped region is an epitaxiallayer.

According to an embodiment, the method further comprises a first dopedregion, wherein the second doped region is sandwiched between the firstdoped region and the sacrificial substrate.

According to an embodiment, the first doped region is heavily doped.

According to an embodiment, the first doped region is an epitaxiallayer.

According to an embodiment, the first and second doped regions form adiode.

According to an embodiment, the electric contact of the chip comprisesstructures extending into the X-ray absorption layer.

BRIEF DESCRIPTION OF FIGURES

FIG. 1A schematically shows a cross-sectional view of the detector,according to an embodiment.

FIG. 1B schematically shows a detailed cross-sectional view of thedetector, according to an embodiment.

FIG. 1C schematically shows an alternative detailed cross-sectional viewof the detector, according to an embodiment.

FIG. 1D schematically shows that the electrical contacts on the X-rayabsorption layer may have structures extending into the X-ray absorptionlayer.

FIG. 2 schematically shows that the device may have an array of pixels,according to an embodiment.

FIG. 3A schematically shows the electronics layer, according to anembodiment.

FIG. 3B schematically shows the electronics layer, according to anembodiment.

FIG. 3C schematically shows the electronics layer, according to anembodiment.

FIG. 4A shows a top view of the RDL in FIG. 3A, according to anembodiment.

FIG. 4B shows a top view of the RDL in FIG. 3A, according to anembodiment.

FIGS. 5A-5D schematically show a flow of making the X-ray absorptionlayer, according to an embodiment.

FIGS. 6A-6D schematically show a flow of making the X-ray absorptionlayer, according to an embodiment.

FIG. 7A shows that multiple chips may be bonded to a single electroniclayer, where each chip may include an X-ray absorption layer, accordingto an embodiment.

FIG. 7B shows that a single X-ray absorption layer may be bonded to asingle electronic layer, according to an embodiment.

FIG. 8 schematically shows a system comprising the semiconductor X-raydetector described herein, suitable for medical imaging such as chestX-ray radiography, abdominal X-ray radiography, etc., according to anembodiment

FIG. 9 schematically shows a system comprising the semiconductor X-raydetector described herein suitable for dental X-ray radiography,according to an embodiment.

FIG. 10 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector describedherein, according to an embodiment.

FIG. 11 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detectordescribed herein, according to an embodiment.

FIG. 12 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector described herein, according to anembodiment.

FIG. 13 schematically shows an X-ray computed tomography (X-ray CT)system comprising the semiconductor X-ray detector described herein,according to an embodiment.

FIG. 14 schematically shows an electron microscope comprising thesemiconductor X-ray detector described herein, according to anembodiment.

DETAILED DESCRIPTION

FIG. 1A schematically shows a cross-sectional view of the detector 100,according to an embodiment. The detector 100 may include an X-rayabsorption layer 110 and an electronics layer 120 (e.g., an ASIC) forprocessing or analyzing electrical signals incident X-ray generates inthe X-ray absorption layer 110. In an embodiment, the detector 100 doesnot comprise a scintillator. The X-ray absorption layer 110 may includea semiconductor material such as, silicon, germanium, GaAs, CdTe,CdZnTe, or a combination thereof. The semiconductor may have a high massattenuation coefficient for the X-ray energy of interest.

As shown in a detailed cross-sectional view of the detector 100 in FIG.1B, according to an embodiment, the X-ray absorption layer 110 mayinclude one or more diodes (e.g., p-i-n or p-n) formed by a first dopedregion 111, one or more discrete regions 114 of a second doped region113. The second doped region 113 may be separated from the first dopedregion 111 by an optional the intrinsic region 112. The discreteportions 114 are separated from one another by the first doped region111 or the intrinsic region 112. The first doped region 111 and thesecond doped region 113 have opposite types of doping (e.g., region 111is p-type and region 113 is n-type, or region 111 is n-type and region113 is p-type). In the example in FIG. 1B, each of the discrete regions114 of the second doped region 113 forms a diode with the first dopedregion 111 and the optional intrinsic region 112. Namely, in the examplein FIG. 1B, the X-ray absorption layer 110 has a plurality of diodeshaving the first doped region 111 as a shared electrode. The first dopedregion 111 may also have discrete portions.

When an X-ray photon hits the X-ray absorption layer 110 includingdiodes, the X-ray photon may be absorbed and generate one or more chargecarriers by a number of mechanisms. An X-ray photon may generate 10 to100000 charge carriers. The charge carriers may drift to the electrodesof one of the diodes under an electric field. The field may be anexternal electric field. The electrical contact 119B may includediscrete portions each of which is in electrical contact with thediscrete regions 114. In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single X-rayphoton are not substantially shared by two different discrete regions114 (“not substantially shared” here means less than 2%, less than 0.5%,less than 0.1%, or less than 0.01% of these charge carriers flow to adifferent one of the discrete regions 114 than the rest of the chargecarriers). Charge carriers generated by an X-ray photon incident aroundthe footprint of one of these discrete regions 114 are not substantiallyshared with another of these discrete regions 114. A pixel 150associated with a discrete region 114 may be an area around the discreteregion 114 in which substantially all (more than 98%, more than 99.5%,more than 99.9%, or more than 99.99% of) charge carriers generated by anX-ray photon incident therein flow to the discrete region 114. Namely,less than 2%, less than 1%, less than 0.1%, or less than 0.01% of thesecharge carriers flow beyond the pixel.

As shown in an alternative detailed cross-sectional view of the detector100 in FIG. 1C, according to an embodiment, the X-ray absorption layer110 may include a resistor of a semiconductor material such as, silicon,germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does notinclude a diode. The semiconductor may have a high mass attenuationcoefficient for the X-ray energy of interest.

When an X-ray photon hits the X-ray absorption layer 110 including aresistor but not diodes, it may be absorbed and generate one or morecharge carriers by a number of mechanisms. An X-ray photon may generate10 to 100000 charge carriers. The charge carriers may drift to theelectrical contacts 119A and 119B under an electric field. The field maybe an external electric field. The electrical contact 119B includesdiscrete portions. In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single X-rayphoton are not substantially shared by two different discrete portionsof the electrical contact 119B (“not substantially shared” here meansless than 2%, less than 0.5%, less than 0.1%, or less than 0.01% ofthese charge carriers flow to a different one of the discrete portionsthan the rest of the charge carriers). Charge carriers generated by anX-ray photon incident around the footprint of one of these discreteportions of the electrical contact 119B are not substantially sharedwith another of these discrete portions of the electrical contact 119B.A pixel 150 associated with a discrete portion of the electrical contact119B may be an area around the discrete portion in which substantiallyall (more than 98%, more than 99.5%, more than 99.9% or more than 99.99%of) charge carriers generated by an X-ray photon incident therein flowto the discrete portion of the electrical contact 119B. Namely, lessthan 2%, less than 0.5%, less than 0.1%, or less than 0.01% of thesecharge carriers flow beyond the pixel associated with the one discreteportion of the electrical contact 119B.

The electronics layer 120 may include an electronic system 121 suitablefor processing or interpreting signals generated by X-ray photonsincident on the X-ray absorption layer 110. The electronic system 121may include an analog circuitry such as a filter network, amplifiers,integrators, and comparators, or a digital circuitry such as amicroprocessors, and memory. The electronic system 121 may includecomponents shared by the pixels or components dedicated to a singlepixel. For example, the electronic system 121 may include an amplifierdedicated to each pixel and a microprocessor shared among all thepixels. The electronic system 121 may be electrically connected to thepixels by vias 131. Space among the vias may be filled with a fillermaterial 130, which may increase the mechanical stability of theconnection of the electronics layer 120 to the X-ray absorption layer110. Other bonding techniques are possible to connect the electronicsystem 121 to the pixels without using vias. The electronic system 121may be configured to count X-ray photons by the pixels or configured tomeasure the amounts of charge carriers accumulated at the pixels (e.g.,by using an analog-to-digital converter (ADC) shared by the pixels).

FIG. 1D schematically shows that the electrical contacts 119A and 119Bmay have structures extending into the X-ray absorption layer 110. Forexample, the structures may be holes are drilled into the X-rayabsorption layer 110 (e.g., by deep reactive-ion etching (DRIE) or laserand filled with a metal. The structures may form an Ohmic contact or aSchottky contact with the materials of the X-ray absorption layer 110.The structures of the electrical contacts 119A and the structures of theelectrical contacts 119B may form an interdigitate pattern but shouldnot electrically short. These structures may help collecting chargecarriers generated from an X-ray photon. The charge carriers only needto drift to one of these structures rather than the surfaces of theX-ray absorption layer 110, thereby reducing the chance of recombinationor trapping. Each of the structures of the electrical contacts 119B maybe spaced by a short distance (e.g., 20 μm, 50 μm or 100 μm) from thenearest one of the structures of the electrical contacts 119A. The timefor the charge carriers to be collected by these structures may be onthe order of 0.1-1 ns.

FIG. 2 schematically shows that the detector 100 may have an array ofpixels 150. The array may be a rectangular array, a honeycomb array, ahexagonal array or any other suitable array. Each pixel 150 may beconfigured to detect an X-ray photon incident thereon, measure theenergy of the X-ray photon, or both. For example, each pixel 150 may beconfigured to count numbers of X-ray photons incident thereon whoseenergy falls in a plurality of bins, within a period of time. All thepixels 150 may be configured to count the numbers of X-ray photonsincident thereon within a plurality of bins of energy within the sameperiod of time. Each pixel 150 may have its own analog-to-digitalconverter (ADC) configured to digitize an analog signal representing theenergy of an incident X-ray photon into a digital signal. The ADC mayhave a resolution of 10 bits or higher. Each pixel 150 may be configuredto measure its dark current, such as before or concurrently with eachX-ray photon incident thereon. Each pixel 150 may be configured todeduct the contribution of the dark current from the energy of the X-rayphoton incident thereon. The pixels 150 may be configured to operate inparallel. For example, when one pixel 150 measures an incident X-rayphoton, another pixel 150 may be waiting for an X-ray photon to arrive.The pixels 150 may be but do not have to be individually addressable.

FIG. 3A schematically shows the electronics layer 120 according to anembodiment. The electronic layer 120 comprises a substrate 122 having afirst surface 124 and a second surface 128. A “surface” as used hereinis not necessarily exposed, but can be buried wholly or partially. Theelectronic layer 120 comprises one or more electric contacts 125 on thefirst surface 124. The one or more electric contacts 125 may beconfigured to be electrically connected to one or more electricalcontacts 119B of the X-ray absorption layer 110. The electronics system121 may be in or on the substrate 122. The electronic layer 120comprises one or more vias 126 extending from the first surface 124 tothe second surface 128. The electronic layer 120 may comprise aredistribution layer (RDL) 123 on the second surface 128. The RDL 123may comprise one or more transmission lines 127. The electronics system121 is electrically connected to the electric contacts 125 and thetransmission lines 127 through the vias 126.

The substrate 122 may be a thinned substrate. For example, the substratemay have at thickness of 750 microns or less, 200 microns or less, 100microns or less, 50 microns or less, 20 microns or less, or 5 microns orless. The substrate 122 may be a silicon substrate or a substrate orother suitable semiconductor or insulator. The substrate 122 may beproduced by grinding a thicker substrate to a desired thickness.

The one or more electric contacts 125 may be a layer of metal or dopedsemiconductor. For example, the electric contacts 125 may be gold,copper, platinum, palladium, doped silicon, etc.

The vias 126 pass through the substrate 122 and electrically connectelectrical components (e.g., the electrical contacts 125) on the firstsurface 124 to electrical components (e.g., the RDL) on the secondsurface 128. The vias 126 are sometimes referred to as “through-siliconvias” although they may be fabricated in substrates of materials otherthan silicon.

The RDL 123 may comprise one or more transmission lines 127. Thetransmission lines 127 electrically connect electrical components (e.g.,the vias 126) in the substrate 122 to bonding pads at other locations onthe substrate 122. The transmission lines 127 may be electricallyisolated from the substrate 122 except at certain vias 126 and certainbonding pads. The transmission lines 127 may be a material (e.g., Al)with small mass attenuation coefficient for the X-ray energy ofinterest. The RDL 123 may redistribute electrical connections to moreconvenient locations. The RDL 123 is especially useful when the detector100 has a large number of pixels. If the detector 100 does not have alarge number of pixels, the RDL 123 may be omitted and signals from thepixels may be routed on the first surface 124.

FIG. 3A further schematically shows bonding between the X-ray absorptionlayer 110 and the electronic layer 120 at the electrical contact 119Band the electrical contacts 125. The bonding may be by a suitabletechnique such as direct bonding or flip chip bonding.

Direct bonding is a wafer bonding process without any additionalintermediate layers (e.g., solder bumps). The bonding process is basedon chemical bonds between two surfaces. Direct bonding may be atelevated temperature but not necessarily so.

Flip chip bonding uses solder bumps 199 deposited onto contact pads(e.g., the electrical contact 119B of the X-ray absorption layer 110 orthe electrical contacts 125). Either the X-ray absorption layer 110 orthe electronic layer 120 is flipped over and the electrical contact 119Bof the X-ray absorption layer 110 are aligned to the electrical contacts125. The solder bumps 199 may be melted to solder the electrical contact119B and the electrical contacts 125 together. Any void space among thesolder bumps 199 may be filled with an insulating material.

FIG. 3B schematically shows the electronics layer 120 according to anembodiment. The electronics layer 120 shown in FIG. 3B is different fromthe electronics layer 120 shown in FIG. 3A in the following ways. Theelectronics system 121 is buried in the substrate 122. The electroniclayer 120 comprises one or more vias 126A extending from the firstsurface 124 to the second surface 128. The vias 126A electricallyconnect the electrical contacts 125 to the transmission lines 127 in theRDL 123 on the second surface 128. The electronic layer 120 furthercomprises one or more vias 126B extending from the second surface 128 tothe electronics system 121. The vias 126B electrically connect thetransmission lines 127 to the electronics system 121. The X-rayabsorption layer 110 and the electronic layer 120 may also be bondedtogether (e.g., at the electrical contact 119B and the electricalcontacts 125) by a suitable technique such as direct bonding or flipchip bonding.

FIG. 3C schematically shows the electronics layer 120 according to anembodiment. The electronics layer 120 shown in FIG. 3C is different fromthe electronics layer 120 shown in FIG. 3A in the following ways. Theelectronics system 121 is buried in the substrate 122. The electroniclayer 120 does not comprise one or more electric contacts 125 on thefirst surface 124. Instead, the substrate 122 including the buriedelectronics system 121 is bonded to the X-ray absorption layer 110 bydirect bonding. Holes are formed in the substrate 123 and filled withmetal to form the vias 126A that electrically route the electricalcontact 119B to the second surface 128 and to form the vias 126B thatelectrically route the electronics system 121 to the second surface 128.The RDL 123 is then formed on the second surface 128 such that thetransmission lines 127 electrically connect the vias 126A and 126B tocomplete the electrical connection from the electrical contact 119B tothe electronics system 121. The X-ray absorption layer 110 may includemultiple discrete chips. Each of the chips may be bonded to theelectronic layer 120 individually or collectively. The X-ray absorptionlayer 110 including multiple discrete chips may help to accommodate thedifference between the thermal expansion coefficients of the materialsof the X-ray absorption layer 110 and the electronic layer 120.

Signal from the electric contacts 125 may be read out column by column.For example, signal from one electric contact 125 may be stored inregister in the electronics system 121 associated with it; the signalmay be successively shifted from one column to the next, and eventuallyto other processing circuitry. If the RDL 123 exists, FIG. 4A shows atop view of the RDL 123 in FIG. 3A to illustrate the positions of thevias 125 and the transmission lines 127, relative to the electriccontacts 125 and the electronics system 121, according to an embodiment.The electric contacts 125, the electronics system 121 and thetransmission lines 127 are shown in dotted lines because they are notdirectly visible in this view.

Signal from the electric contacts 125 may be read out pixel by pixel.For example, signal from one electric contact 125 may be stored inregister in the electronics system 121 associated with it; the signalmay be successively shifted from one electric contact 125 to the next,and eventually to other processing circuitry. If the RDL 123 exists,FIG. 4B shows a top view of the RDL 123 in FIG. 3A to illustrate thepositions of the vias 125 and the transmission lines 127, relative tothe electric contacts 125 and the electronics system 121, according toan embodiment. The electric contacts 125, the electronics system 121 andthe transmission lines 127 are shown in dotted lines because they arenot directly visible in this view.

FIGS. 5A-5D schematically show a flow of making the X-ray absorptionlayer 110, according to an embodiment. FIG. 5A schematically shows thatthe flow starts with a substrate 199 having a sacrificial layer 187, anetch stop layer 188, the first doped region 111, the second doped region113 and the intrinsic region 112. The regions 111, 112 and 113 aredescribed above. The regions 112 and 113, and the optional region 111may function as the X-ray absorption layer 110. The regions 111, 112 and113 may be layers epitaxially grown on the etch stop layer 188. In oneexample, the sacrificial layer 187 is a GaAs wafer. The second dopedregion 113 is an N type GaAs epitaxial layer, which may have a thicknessof about 5 microns. The intrinsic region 112 may be an intrinsic GaAsepitaxial layer with a thickness of less than 75 microns, less than 100microns, less than 125 microns, or less than 200 microns. Alternatively,the intrinsic region 112 may be replaced with a lightly doped P typelayer or a lightly doped N type layer. “Lightly doped” means that theenergy levels of the dopants do not merge into an impurity band. Incontrast, “heavily doped” means that the energy levels of the dopantsmerge into an impurity band. The first doped region 111 may be a heavilydoped P type GaAs epitaxial layer with a thickness of 1 micron or more.In an embodiment, the first doped region 111 may be omitted.

FIG. 5B schematically shows that the substrate 199 is attached to asubstrate 900. There may be a metal layer 930 to form electrical contactto the first doped region 111 if the first doped region 111 is present,or to the intrinsic region 112 if the first doped region 111 is absent.

The substrate 900 may be a material that has a low (e.g., <1000 m²/kg)mass attenuation coefficient for the X-ray energy of interest. Examplesof such a material may include silicon, silicon oxide, Al, Cr, Ti, etc.The substrate 900 does not have to be a single material. In one example,the substrate 900 may include a body of silicon and the surfacecontacting the X-ray absorption layer 110 may be a metal layer. Inanother example, the substrate 900 is a silicon wafer and the surfacecontacting the X-ray absorption layer 110 is heavily doped silicon. Inanother example, the substrate 900 is a glass wafer and the surfacecontacting the X-ray absorption layer 110 is a metal layer. Thesubstrate 900 may have a sufficient strength to provide mechanicalsupport to the X-ray absorption layer 110 during subsequent fabricationprocesses. The surface contacting the X-ray absorption layer 110 may bean electrically conductive material such as heavily doped silicon, Al,Cr, Ti, etc. The substrate 900 may be configured to be electricallyconnected to or serve as the electrical contacts 119A of the X-rayabsorption layer 110.

FIG. 5C schematically shows that the sacrificial layer 187 is removed byetching. Etching is stopped by the etch stop layer 188. Even if theregions 111-113 may be of the same material as the sacrificial layer187, the etch stop layer 188 prevents etching of the regions 111-113while allows etching of the sacrificial layer 187. The etch stop layer188 may be subsequently removed.

FIG. 5D schematically shows that the discrete regions 114 and theelectrical contacts 119B may be formed partially from the second dopedregion 113. The electronics layer 120 may then be attached to the chip189 as described above.

FIGS. 6A-6D schematically show a flow of making the X-ray absorptionlayer 110, according to an embodiment. FIG. 6A schematically shows thatthe flow may start with a substrate 199 having an intrinsic or lighteddoped layer 184 and optionally the first doped region 111. The region111 is described above. In one example, the layer 184 is a GaAs wafer.The layer 184 may be intrinsic or lighted doped P type GaAs. The firstdoped region 111 may be a heavily doped P type GaAs epitaxial layer witha thickness of 1 micron or more. In an embodiment, the first dopedregion 111 may be omitted.

FIG. 6B schematically shows that the chip 189 is attached to a substrate900. There may be a metal layer 930 to form electrical contact to thefirst doped region 111 if the first doped region 111 is present, or tothe layer 184 if the first doped region 111 is absent.

The substrate 900 may be a material that has a low (e.g., <1000 m²/kg)mass attenuation coefficient for the X-ray energy of interest. Examplesof such a material may include silicon, silicon oxide, Al, Cr, Ti, etc.The substrate 900 does not have to be a single material. In one example,the substrate 900 may include a body of silicon and the surfacecontacting the X-ray absorption layer 110 may be a metal layer. Inanother example, the substrate 900 is a silicon wafer and the surfacecontacting the X-ray absorption layer 110 is heavily doped silicon. Inanother example, the substrate 900 is a glass wafer and the surfacecontacting the X-ray absorption layer 110 is a metal layer. Thesubstrate 900 may have a sufficient strength to provide mechanicalsupport to the X-ray absorption layer 110 during subsequent fabricationprocesses. The surface contacting the X-ray absorption layer 110 may bean electrically conductive material such as heavily doped silicon, Al,Cr, Ti, etc. The substrate 900 may be configured to be electricallyconnected to or serve as the electrical contacts 119A of the X-rayabsorption layer 110.

FIG. 6C schematically shows that the layer 184 is thinned by a suitablemethod such as grinding, to a suitable thickness of less than 75microns, less than 100 microns, less than 125 microns, or less than 200microns.

FIG. 6D schematically shows that the second doped region 113 may bedisposed onto the layer 184. The layer 184, the second doped region 113,and the optional first doped region 111 may function as the X-rayabsorption layer 110. The discrete regions 114 and the electricalcontacts 119B may be formed partially from the second doped region 113.The electronics layer 120 may then be attached to the chip 189 asdescribed above.

The X-ray absorption layer 110 as made by the flow of FIGS. 5A-5D or theflow of FIGS. 6A-6D may be bonded to the electronic layer 120 using asuitable method such that in FIG. 3A, 3B or 3C.

FIG. 7A shows that multiple chips may be bonded to a single electroniclayer 120, where each chip may include an X-ray absorption layer 110,according to an embodiment. The smaller sizes of the chips relative tothe electronic layer 120 may help accommodating the difference inthermal expansion coefficients of the chips and the electronic layer120. A ratio between the thermal expansion coefficient of the chips andthe thermal expansion coefficient of the electronic layer 120 may be twoor more. Bonding multiple chips onto a single electronic layer 120 mayyield a large area detector.

FIG. 7B shows that a single X-ray absorption layer 110 may be bonded toa single electronic layer 120, according to an embodiment. FIG. 7B isespecially suitable for applications (e.g., intraoral X-rayapplications) where the size of the detector does not have to be large.

FIG. 8 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as chest X-ray radiography, abdominal X-ray radiography,etc. The system comprises an X-ray source 1201. X-ray emitted from theX-ray source 1201 penetrates an object 1202 (e.g., a human body partsuch as chest, limb, abdomen), is attenuated by different degrees by theinternal structures of the object 1202 (e.g., bones, muscle, fat andorgans, etc.), and is projected to the semiconductor X-ray detector 100.The semiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the X-ray.

FIG. 9 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as dental X-ray radiography. The system comprises an X-raysource 1301. X-ray emitted from the X-ray source 1301 penetrates anobject 1302 that is part of a mammal (e.g., human) mouth. The object1302 may include a maxilla bone, a palate bone, a tooth, the mandible,or the tongue. The X-ray is attenuated by different degrees by thedifferent structures of the object 1302 and is projected to thesemiconductor X-ray detector 100. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of the X-ray.Teeth absorb X-ray more than dental caries, infections, periodontalligament. The dosage of X-ray radiation received by a dental patient istypically small (around 0.150 mSv for a full mouth series).

FIG. 10 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector 100 describedherein. The system may be used for inspecting and identifying goods intransportation systems such as shipping containers, vehicles, ships,luggage, etc. The system comprises an X-ray source 1401. X-ray emittedfrom the X-ray source 1401 may backscatter from an object 1402 (e.g.,shipping containers, vehicles, ships, etc.) and be projected to thesemiconductor X-ray detector 100. Different internal structures of theobject 1402 may backscatter X-ray differently. The semiconductor X-raydetector 100 forms an image by detecting the intensity distribution ofthe backscattered X-ray and/or energies of the backscattered X-rayphotons.

FIG. 11 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detector 100described herein. The system may be used for luggage screening at publictransportation stations and airports. The system comprises an X-raysource 1501. X-ray emitted from the X-ray source 1501 may penetrate apiece of luggage 1502, be differently attenuated by the contents of theluggage, and projected to the semiconductor X-ray detector 100. Thesemiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the transmitted X-ray. The system may revealcontents of luggage and identify items forbidden on publictransportation, such as firearms, narcotics, edged weapons, flammables.

FIG. 12 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector 100 described herein. The full-body scannersystem may detect objects on a person's body for security screeningpurposes, without physically removing clothes or making physicalcontact. The full-body scanner system may be able to detect non-metalobjects. The full-body scanner system comprises an X-ray source 1601.X-ray emitted from the X-ray source 1601 may backscatter from a human1602 being screened and objects thereon, and be projected to thesemiconductor X-ray detector 100. The objects and the human body maybackscatter X-ray differently. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of thebackscattered X-ray. The semiconductor X-ray detector 100 and the X-raysource 1601 may be configured to scan the human in a linear orrotational direction.

FIG. 13 schematically shows an X-ray computed tomography (X-ray CT)system. The X-ray CT system uses computer-processed X-rays to producetomographic images (virtual “slices”) of specific areas of a scannedobject. The tomographic images may be used for diagnostic andtherapeutic purposes in various medical disciplines, or for flawdetection, failure analysis, metrology, assembly analysis and reverseengineering. The X-ray CT system comprises the semiconductor X-raydetector 100 described herein and an X-ray source 1701. Thesemiconductor X-ray detector 100 and the X-ray source 1701 may beconfigured to rotate synchronously along one or more circular or spiralpaths.

FIG. 14 schematically shows an electron microscope. The electronmicroscope comprises an electron source 1801 (also called an electrongun) that is configured to emit electrons. The electron source 1801 mayhave various emission mechanisms such as thermionic, photocathode, coldemission, or plasmas source. The emitted electrons pass through anelectronic optical system 1803, which may be configured to shape,accelerate, or focus the electrons. The electrons then reach a sample1802 and an image detector may form an image therefrom. The electronmicroscope may comprise the semiconductor X-ray detector 100 describedherein, for performing energy-dispersive X-ray spectroscopy (EDS). EDSis an analytical technique used for the elemental analysis or chemicalcharacterization of a sample. When the electrons incident on a sample,they cause emission of characteristic X-rays from the sample. Theincident electrons may excite an electron in an inner shell of an atomin the sample, ejecting it from the shell while creating an electronhole where the electron was. An electron from an outer, higher-energyshell then fills the hole, and the difference in energy between thehigher-energy shell and the lower energy shell may be released in theform of an X-ray. The number and energy of the X-rays emitted from thesample can be measured by the semiconductor X-ray detector 100.

The semiconductor X-ray detector 100 described here may have otherapplications such as in an X-ray telescope, X-ray mammography,industrial X-ray defect detection, X-ray microscopy or microradiography,X-ray casting inspection, X-ray non-destructive testing, X-ray weldinspection, X-ray digital subtraction angiography, etc. It may besuitable to use this semiconductor X-ray detector 100 in place of aphotographic plate, a photographic film, a PSP plate, an X-ray imageintensifier, a scintillator, or another semiconductor X-ray detector.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. An apparatus comprising: an X-ray absorptionlayer; a first electrical contact and a second electrical contact onopposing surfaces of the X-ray absorption layer; wherein the firstelectrical contact and the second electrical contact respectivelycomprise structures extending into the X-ray absorption layer.
 2. Theapparatus of claim 1, wherein the structures are holes filled with ametal.
 3. The apparatus of claim 1, wherein the structures form an Ohmiccontact with materials of the X-ray absorption layer.
 4. The apparatusof claim 1, wherein the structures form a Schottky contact withmaterials of the X-ray absorption layer.
 5. The apparatus of claim 1,wherein the structures of the first electrical contact and thestructures of the second electrical contact form an interdigitatepattern.
 6. The apparatus of claim 1, wherein the structures of thefirst electrical contact and the structures of the second electricalcontact do not electrically short.
 7. The apparatus of claim 1, whereineach of the structures of the first electrical contact is spaced by adistance less than 100 μm from the nearest one of the structures of thesecond electrical contact.
 8. The apparatus of claim 1, wherein thestructures of the first electrical contact and the structures of thesecond electrical contact are configured to collect charge carriersgenerated in the X-ray absorption layer from X-ray photons.
 9. Anapparatus comprising: an X-ray absorption layer; an electronics layercomprising a substrate and an electronic system; wherein the electronicsystem is configured to process or interpret signals generated by X-rayphotons incident on the X-ray absorption layer, and is buried in thesubstrate; wherein the substrate has a first surface and a secondsurface opposing the first surface; wherein the electronics layercomprises a redistribution layer (RDL) on the second surface, the RDLcomprising transmission lines; wherein the electronics layer comprisesfirst vias extending from the first surface to the second surface, andelectrically connected to the transmission lines; wherein theelectronics layer comprises second vias extending from the secondsurface, and electrically connecting the transmission lines to theelectronic system.
 10. The apparatus of claim 9, wherein the electronicslayer comprises electric contacts on the first surface; wherein thefirst vias are electrically connected to the electric contacts.
 11. Amethod comprising: bonding an X-ray absorption layer having electricalcontacts to a substrate having a first surface and a second surfaceopposing the first surface, so that the electrical contacts are on thefirst surface of the substrate; forming first vias extending from thefirst surface to the second surface and electrically connected to theelectrical contacts; forming second vias extending from the secondsurface and electrically connected to an electric system buried in thesubstrate; forming a redistribution layer (RDL) on the second surface,the RDL comprising transmission lines connected to the first vias andthe second vias.
 12. The method of claim 11, wherein forming the firstvias or the second vias comprises forming holes in the substrate andfilling the holes with a metal.
 13. The method of claim 11, wherein thesubstrate comprises electric contacts on the first surface; wherein thefirst vias are electrically connected to the electric contacts.